This invention relates to a method for recovering sublimable material such as pyromellitic dianhydride and terephthalic acid at an improved efficiency.
Pyromellitic dianhydride, which is one of sublimable materials, is useful as raw material for heat resistance polymers such as polyimide resin and as a curing agent for epoxy resin. Known as a method for producing pyromellitic dianhydride at a high purity has been a method disclosed in Japanese Examined Patent Publication No. Sho 47-18745. In this known method, pyromellitic dianhydride is deposited from an reaction gas containing pyromellitic dianhydride produced by the vapor phase oxidation method. Specifically, a reaction gas containing pyromellitic dianhydride is introduced into a recovery apparatus which is formed with a depository surface and kept at a temperature lower than a subliming temperature of pyromellitic dianhydride, thereby rendering pyromellitic dianhydride to deposit on the depository surface in the form of crystal. Thereafter, the deposited pyromellitic dianhydride crystal is removed from the depository surface.
Also, Japanese Examined Patent Publication No. Sho 57-27722 discloses a method in which a reaction gas containing pyromellitic dianhydride is allowed to deposit on a cooling plate formed with small holes, and the deposited pyromellitic dianhydride crystal is scraped off by a rotated comb-like or brush-like blade.
Further, Japanese Unexamined Patent Publication No. Hei 4-131101 discloses a method in which a reaction gas containing pyromellitic dianhydride is introduced into a recovery apparatus together with hard-wearing particles, and the deposited pyromellitic dianhydride crystal is removed from a depository surface through collision of particles.
Furthermore, there has been proposed a method of applying a mechanical impact to a recovery apparatus by an air knocker and the like to remove the deposited pyromellitic dianhydride crystal from a depository surface.
Pyromellitic dianhydride is a sublimable substance, and has a melting point of 286.degree. C. and a considerably high subliming temperature. For these reason, it has been difficult to take out deposited pyromellitic dianhydride crystal from a recovery apparatus.
In the case of sublimable material having a low subliming temperature, the sublimable material crystal can be easily obtained by heating a recovery apparatus formed with deposited sublimable material crystal to sublime the deposited sublimable material. However, in the case of sublimable material having a high subliming temperature, it is hard to sublimate the deposited sublimable material. Particularly, in the case of sublimable material having a subliming temperature of 200.degree. C. or higher, a pressure of 1500 KPaG or higher is required to sublime deposited material when using steam. This needs equipment to generate steam. Alternatively, when using an electric heater as heating source, there is the problem of consuming a considerable large energy to sublime deposited material. Further, even if melting deposited sublimable material, a considerable large energy is required. Moreover, in the case of sublimable material being an organic compound, decomposition or deterioration is likely to occur when exceeding a specified temperature, resulting in industrially fatal damage.
On the other hand, in the case of mechanical recovery as the use of scraping blade, disclosed by Japanese Examined Patent Publication No. Sho 57-27722, without heating, there are problems that the recovering construction becomes complicated and driving trouble is liable to occur, and little degree of freedom in designing a recovery apparatus.
Also, in the case of adding hard-wearing particles as disclosed by Japanese Unexamined Patent Publication Patent No. Hei 4-131101, it is necessary to separate hard-wearing particles which are added in reaction gas containing sublimable material introduced into a recovery apparatus. Accordingly, there is the problem that a special apparatus is provided to separate hard-wearing particles, disadvantageously making the size of sublimable material recovery equipment larger.
Further, in the case of applying an mechanical impact to a recovery apparatus with an air knocker, the recovery apparatus is required to have a strength enough to withstand the impact. Also, the impact is unavoidably applied to a limited portion of the recovery paratus. Consequently, deposited sublimable material cannot be uniformly removed.